The invention relates to a device comprising a space which is evacuated or filled with a protective gas, this device having an electron-emitting body which can be coated at an electron-emitting surface from a reservoir with material for reducing the electron work function.
The electron-emitting body may be a thermionic cathode, for example, in a vacuum tube, but may especially be a semiconductor cathode; in the latter case, various kinds of semiconductor cathodes may be used, such as NEA cathodes, field emitters and more particularly reverse junction cathodes, as described in Netherlands Patent Application No. 7905470, in the name of the Applicant, corresponding to U.S. Pat. Nos. 4,303,930 and 4,370,797. Such vacuum tubes are suitable to be used as camera tubes or display tubes, but may also be used in apparatus for Auger spectroscopy, electron microscopy and electron lithography.
The above devices may also be provided with a photocathode, incident radiation leading to an electron current which leaves the photocathode. Such photocathodes are used in photocells, camera tubes, image converters and photomultiplier tubes. Another application of a device according to the invention resides in so-called thermionic converters, in which thermal radiation is converted into an electron current.
The invention further relates to a reservoir for such an arrangement.
Such a device is known from Netherlands Patent Specification No. 18,162, corresponding to U.S. Pat. No. 1,767,437. In this case, caesium is deposited in a discharge tube by heating a dissolved mixture of caesium chloride and barium oxide so that the caesium chloride is reduced by the released barium to metallic caesium, which spreads over the interior of the discharge tube. In an embodiment shown in the Patent Specification, the mixture to be heated is provided in a side tubule attached to the vacuum tube, which afterwards is sealed off from this tube.
In this arrangement, a quantity of caesium is consequently introduced only once into the vacuum space. If use is made of a semiconductor cathode, this caesium will cover the emitting surface as a mono-atomic layer, after which reduction of the quantity of caesium on the emitting surface cannot or can substantially not be compensated. Such a reduction of caesium or another material reducing the electron work function at the surface is due inter alia to desorption and migration under the influence of electric fields and gives rise to degradation of the emission. The ultimate efficiency of, for example, a reverse biased junction cathode thus remains limited to 20 to 40% of the optimum value.